This invention relates to yellow pigment of iron oxide crystals having a particle shape represented by a major axis/minor axis length ratio falling in the range of 1.5 to 4.5 and having a particle size distribution in a narrow range and to a method for the manufacture of the yellow pigment. This invention is directed to providing yellow iron oxide pigment excelling in color tone, dispersibility and thermal resistance and developing this yellow pigment for new uses in thermoprocessing resins, cosmetics, traffic paints, etc.
Yellow pigments with highly desirable properties available today include chrome yellow, strontium yellow, cadmium yellow, and bendizine yellow. Since these yellow pigments are invariably toxic or carcinogenic substances, their use is strictly controlled for the prevention of environmental pollution and for the preservation of human health.
In the coloring material industry, therefore, development of an excellent nontoxic yellow pigment has been desired.
Yellow iron oxide has a composition of .alpha.-FeOOH and a structure of Goethite and has been long used as a coloring material known popularly as goethite, loess, ocher. It shows no toxicity, enjoys weather resistance and stability, and is not expensive. The uses found for this compound cover coloration of paints, printing inks and building materials and, because of its freedom from toxicity, further include coloration of cosmetics, roll papers for tobacco filters and chicken feed. Since the end of the war, it has come to be used as a raw material for the magnetic powder in magnetic recording. The demand for this compound is growing conspicuously. The compound when used as a pigment entails disadvantages that the color tone is slightly unclear, that the viscosity is high due to the acicular shape of the particles and that the thermal resistance is inferior. Because of these inferior properties, the use of this compound as a substitute for the aforementioned toxic yellow pigments has been restricted. Improvement of the properties and enhancement of the quality of this compound as a pigment, therefore, has been desired.
For the manufacture of yellow iron oxide, the method which causes microcystalline cores of iron oxide to grow to a desired particle diameter by utilizing the hydrolysis of an aqueous ferrous sulfate solution and the reaction of air oxidation is now in popular use. For very gently neutralizing the sulfuric acid produced by the hydrolysis of the aqueous ferrous sulfate solution and accelerating the growth of crystals, there are known to the art a method which uses iron filings [Nobuoka et al., Kogyo Kagaku Zasshi 66, 412 (1963)] and a method which makes use of ammonia gas (Yada, Electronic Ceramics, '72, No. 1, p. 15) are known to the art. FIG. 1 and FIG. 2 represent electron photomicrographs, at 40,000 magnifications, of samples of yellow iron oxide produced by the methods described above for commercial sale. Their particles are in the shape of dendritic or acicular tactoids having an axial ratio (minor axis/minor axis) of not less than 5.0. They include giant particles and fine particles. The acicular particles have widely varied axial ratios, with the particle sizes distributed in a wide range. The methods used for their production are based on the growth of crystals in aqueous solutions of insoluble salts. In view of the mechanism responsible for their production, it is considered extremely difficult to obtain crystals of more uniform particle sizes and more uniform quality than are attainable at present and it is considered infeasible to alter the shape and axial ratio of particles by merely adjusting the production conditions involved.
It is widely known that the properties exhibited by a given powdery system are generally interrelated with the morphology of individual particles making up the powdery system. In a pigment, the morphology of its particles has effects upon color tone, hiding power, oil absorption, and tinting strength and upon rheological properties and film strength of the pigment as used in paints. In the case of yellow iron oxide, if it contains giant particles and fine particles at the same time, the properties exhibited by the giant particles and those exhibited by the fine particles are different. The properties which the pigment exhibits as one powdery system are statistical averages of such two sets of properties and such original properties are no longer retained. In the case of color tone, for example, since the color produced by giant particles and the color produced by fine particles are different as described in Nobuoka, Report of the Government Industrial Research Institute, Osaka, No. 331, p. 33 (1969), combination of these particles results in a subtractive mixture as in the mixture of colors. Consequently, both lightness and saturation are lowered and a darkish color tone is obtained. Ideally, therefore, the pigment should be a powder having a particle size distribution in a narrow range. The pigment is further desired to be formed of acicular particles which have a small axial ratio, possess a high dispersing property, and exhibit a low oil absorption.
The inventors continued a diligent study on yellow iron oxide for many years and contributed to the improvement of quality of this compound. The inventions resulting from this study have matured into the following patents and patent publications: Japanese Patent Publication SHO 31(1956)-3292 to Ando and Nobuoka, Japanese Patent Publication SHO 53(1978)-28158) to Nobuoka, Asai and Ado, U.S. Pat. No. 3,969,494 to Nobuoka, Asai and Ado, and Japanese Patent Publication SHO 55(1980)-5016 to Nobuoka et al. These inventions have been being reduced to practice.
To make an excellent pigment, the yellow iron oxide is required to have a small axial (major axis/minor axis) ratio, a narrow range of particle size distribution, and a particle size suitable for the particular purpose.